The field of art to which the invention pertains is catalytic composites. More specifically, the invention relates to a catalytic composite comprising a clay which has been contacted with a strong acid and thereafter partially crystallized.
Hydrocarbon conversion catalysts containing crystalline aluminosilicates (zeolites) have been available for many years. Heretofore, two basic methods of preparing zeolitic containing catalysts have been known. One method involves the mixing of a zeolite, either natural or synthetic, with a matrix such as amorphous silica-alumina hydrogel or clay (Elliot, Jr. et. al., U.S. Pat. No. 3,867,310; Magee, Jr. et. al., U.S. Pat. No. 3,501,418). The other method involves performing particles of a precursor of the desired zeolite structure, and thereafter crystallizing the preformed particles (McDaniel, et. al., U.S. Pat. No. 3,472,617; Haden, Jr. et. al., U.S. Pat. No. 3,657,154).
In the former case, the matrix acts as a binder. The activity of the composite derives mostly from the zeolite crystals admixed with the matrix. One purpose of the matrix is to provide composite catalytic particles of proper size for the particular application. For example, in a fluid catalytic cracking application the catalytic particles must be of such size that they can freely circulate through the fluid catalytic cracking reactor-regenerator system and at the same time be of such size that they are recoverable by cyclones in the regenerator. In addition, zeolites alone are too active for many applications. Because of their high activity the zeolite particles would promote in some applications the formation of excess carbon and high molecular weight compounds which would be deposited on the surface of the zeolite particles. The catalyst would be quickly deactivated as a result. The use of a matrix material allows the manufacture of catalytic composites of proper size with activities lower than those which pure zeolite particles of the same size would display. However, the same properties of the matrix which are advantageous in some circumstances are disadvantageous in others. Admixing a matrix with a zeolite tends to substantially deactivate the zeolite. The deactivation appears to occur because the matrix generally provides no ordered access to reactive zeolite sites. Molecular diffusion of the reactive species to zeolite sites appears to be substantially hindered by even a crystalline matrix material. Thus, although admixture of zeolite and matrices can result in catalytic composites of proper size, the activity level of the resulting composite is lower than has been desired, apparently because much of the zeolite constituent is rendered somewhat inaccessible by the matrix.
Acid treating has been disclosed to improve the pore structure of catalyst composites under some conditions. Particle-form alumina-containing oxides combined with crystalline zeolites have been acid treated to increase their porosity and permeability. (Young, et. al., U.S. Pat. No. 3,836,561) However, many crystalline zeolites are very susceptible to attack by acids, and consequent destruction of the desired crystalline structure. Acid treating of certain clays to alter the pore structure of the clay for use in the preparation of catalyst supports has been disclosed. (Alafandi, U.S. Pat. No. 3,962,135; Alafandi, U.S. Pat. No. 4,142,994) Again, the crystallinity of the clay may be substantially destroyed by the acid treating operation. Acid treating of clays has also been disclosed to increase the acid resistance of composites made using the acid treated clay. (Young, U.S. Pat. No. 3,691,099; Eastwood, U.S. Pat. No. 3,406,124) It appears that simply altering the pore structure of a clay and thereafter admixing the clay with zeolites does not necessarily result in greater accessibility of the zeolites to reactants.
As indicated above, another method which has been disclosed of making hydrocarbon conversion catalysts containing zeolites involves preforming particles of a precursor of the desired crystalline structure, and thereafter crystallizing the precursor particles. Generally, when binderless zeolitic aggregates are formed using starting materials such as silica-alumina and clay, the products have poor attrition resistance. During processing the particles tend to agglomerate, and are subject to mechanical disintegration unless great care is exercised. Typically, the processes are relatively expensive, requiring, for example, a high temperature spray drying step, or high purity starting materials. In addition, the particles are first formed, and then crystallized. The product is a crystal structure with a narrowly limited range of pore sizes, the pore characteristics of which are contributed exclusively by the resulting crystal structure. Another characteristic of crystalline catalysts formed in such a manner is that they are silicate depleted. Because crystallization occurs hydrothermally in an alkaline environment, significant amounts of silicate are removed from the crystal structure, lowering the SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of the final crystalline product. Since a typical starting material, kaolin clay, has a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of 2:1; the structural integrity of the resulting crystal structure is substantially altered by removal of significant amounts of silicate.
What has been needed is a catalytic composite which is derived from clay, can be prepared in powder form or a range of particle sizes, has a relatively uniform distribution of pore volumes over a wide range of pore sizes, a relatively high activity, and a silica to alumina ratio which is at least as great as the clay from which it is derived. The catalytic composite of this invention satisfies these requirements. Further, the catalytic composite of this invention is unexpectedly easy to produce, and can be manufactured less expensively than many other catalytic composites intended for similar use.